There is a need for drilling multiple angled, long reach boreholes from a fixed location such as from an offshore drilling platform. Historically, several methods have been used to change the direction of a borehole. With the requirement for multiple extended reach drilling of wells from offshore platforms came the need for a means for steering the drilling assembly more accurately. In the 1970s, the downhole motor and Measurement-While-Drilling (MWD) with a bent sub were introduced. Steering was accomplished by stopping rotary drilling and installing the downhole motor-bent sub assembly and an orientation tool. After making a trip into the borehole, the orienting tool was actuated and locked into the desired tool face angle—the angle of the assembly at the bottom of the hole similar to the points of a compass. The downhole motor's bent sub (typically with a two-degree bend) is actuated by increasing pump pressure, thus turning the motor and the drill bit. The assembly drills ahead with the drill string sliding forward and only the drill bit rotating, thus increasing the hole build angle approximately 2 degrees per length of the motor until the desired angle is achieved. It is during the sliding advancement of the drill string that differential sticking (a significant and frequently incurred problem) is most prevalent. The downhole motor is retrieved, thus requiring another trip to the surface. In later designs, after drilling the build section and when a short straight hole section is required, a trip to the surface can be delayed by rotating the bent sub downhole motor at drilling speeds (5-150 RPM) until the short straight section is drilled. This method can drill an approximately straight but slightly enlarged hole for short distances. The amount of time between trips is typically limited by the life of the downhole motor (80-100 hours), rather than the life of the bit (the preferred condition) which can be as high as 350-400 hours.
Thus, drilling with a downhole motor and a bent sub has disadvantages of being expensive and time consuming because of the trips in and out of the borehole when steering to each desired new angle, and this approach is unreliable because the downhole motor has a greater tendency to break down under these conditions.
Later, steering tools that were directly attached to the drill string were developed. Modern steering tools of this type are either discrete or integrated. Discrete steering tools include Halliburton's TRACS 2D, Maersk's “wall grabber” style tool, Directional Drilling Dynamics' tool that rotates through a bend, and the Cambridge Radiation tool that includes a non-rotating body that deflects the drill string.
Integrated steering tools are part of an assembly of other downhole tools including downhole sensors. Suppliers of these include Halliburton's TRACS 2D, Smith Red Barron which includes a non-rotating near bit stabilizer (Wall Grabber), and the ANADRILL tool that is being integrated into a Camco tool. Baker Hughes Inteq has the AUTO TRAK tool that includes directional resistivity and vibration measurements. Camco has a 3-D SRD tool with sensors that can perform five jobs without a major overhaul.
Certain prior art steering tools can change azimuth and inclination simultaneously. These tools, one of which is manufactured by Schlumberger, utilizes three pistons which extend laterally outwardly from the drill string at different distances to push the drill string off center to change orientation of the drill string. This approach avoids use of a bent sub. However, use of pistons in a small diameter drill hole to make steering adjustments is not desirable; and they are costly and less reliable because of the large number of mechanical parts.
The previously mentioned MWD system is a separate standalone assembly comprising survey equipment which uses an inclinometer or accelerometer for measuring inclination and a magnetometer for measuring azimuth angle. Inclination angle is typically measured away from vertical (90 degrees from the horizontal plane), and azimuth angle is measured as a rotational angle in a horizontal plane, with magnetic North at zero degrees and West at 270 degrees, for example.
There is a need for a low cost, highly reliable, long life three-dimensional rotary drilling tool that provides steering in both azimuth and inclination while drilling. It is also desirable to provide a steering tool which can change both inclination and azimuth angles without use of a downhole motor and bent sub and the time consuming and expensive trips to the surface for changing orientation of the steering tool. It would also be desirable to avoid use of wall grabber type systems that require contact with the wall of the borehole to push the drill string off center in order to change drilling angles.
The present invention provides a steering tool which can change inclination and azimuth angles either continuously (simultaneously) or incrementally while rotary drilling and while making such steering adjustments in three dimensions. Changes in inclination and azimuth while rotary drilling can be made with drilling fluid flowing through the drill string and up the bore. The steering assembly of this invention can respond to electrical signals via onboard mud pulse telemetry to control the relative azimuth and inclination angles throughout the drilling process. Such three dimensional steering can be achieved without stopping the drilling process, without use of a downhole motor or bent sub, and without borehole wall contacting devices that externally push the drill string toward a desired orientation. The invention provides a steering tool having lower cost, greater reliability, and longer life than the steering tools of the prior art, combined with the ability to improve upon long reach angular drilling in three dimensions with reduced torque and drag.